docs: def cmd(self):
          """
          Determine the final CMD instruction, if any, in the final build stage.
          CMDs from earlier stages are ignored.
          :return: value of final stage CMD instruction
          """
          value = None
          for insndesc in self.structure:
              if insndesc['instruction'] == 'FROM':  # new stage, reset
                  value = None
              elif insndesc['instruction'] == 'CMD':
                  value = insndesc['value']
          return value